Until a few years ago the known forms of carbon were graphite, diamond, and graphite-like particles called amorphous carbon. Then in 1985 another form of carbon was discovered: a hollow cluster of 60 carbon atoms shaped like a soccer ball. This molecule also became known as a “Buckminsterfullerene” (or a “fullerene” for short). The name is in recognition of the American architect R. Buckminster Fuller, whose geodesic domes have a similar structure. Carbon nanotubes were discovered in 1991. Carbon nanotubes are cylindrical, stretched versions of hollow fullerenes. Some nanotubes have walls that are a single carbon atom thick; others have two or more concentric layers of atoms. Because of the 60 carbon atoms, fullerenes are sometimes referred to as C60. Carbon also forms other molecular structures, such as C70, C76, C84, and C102. All of these forms of carbon typically only exist as very small structures having at least one physical dimension that is smaller than 100 nanometers. These materials are collectively referred to as carbon “nanostructures.” The term nanostructures encompasses nanotubes, nanoparticles and other nanometer-size materials. Carbon nanostructures have been an area of significant interest because of their unusual electrical and mechanical properties. In addition to carbon nanostructures, other forms of nanostructures are silicon nanoparticles, silicon nanofibers, silicon-based nanostructured materials, and rare earth or metal-doped silicon nanostructured materials, as well as boron and germanium nanostructures. Nanostructures offer promise in such applications as superstrong materials, extremely small and fast computer chips, and electronic interconnects.
One major obstacle to commercial development of nanotechnology is the inefficiency of production processes for manufacturing nanostructured materials. Current state-of-the-art manufacturing processes are very limited in capacity, and alternative methods that have been proposed are not economically viable. Methods that are typically used to manufacture carbon nanomaterials include electric arc, laser or chemical conversion processes that use a gas precursor such as alcohols (e.g., ethanol, methanol), carbon monoxide, methane, or ethyne (acetylene) as the feedstock or use a solid material that is vaporized by one of these processes. A specific difficulty that is often encountered with these processes is that gas boundary layers on the collection surfaces prevent a high growth rate of nanostructures.
Another impediment to commercial development of nanotechnology is that current production methods for nanostructured materials, particularly carbon nanotubes, result in either (a) a very low percentage of single wall or multi-wall nanotubes mixed with large amounts of precursor or byproduct materials, or (b) a mixture of single wall and multi-wall nanotubes. The extraction of single wall or multi-wall nanotubes from extraneous material is costly and time consuming, and the separation of single wall from multi-wall nanotubes is extremely difficult. Consequently, these present processes typically produce only a few grams of material a day. The resulting high costs of producing nanostructures severely limits their use in commercial products, and consequently these materials have generally been relegated to the realm of a scientific curiosity. What are needed are production apparatuses and techniques for economically manufacturing high purity nanostructured materials in high volumes.